1. Field of The Invention
The present invention relates generally to an apparatus for detecting header information recorded on a header of each sector for a magnetic disc recording medium.
2. Description of The Background Art
Generally, on a disc recording medium such as an optical-magnetic disc, each sector includes a header portion and a data region. The header is constituted by an address mark and address information such as a track number and sector number. A head position of the address information is identified by an address mark.
For example, to improve the information reliability, the address information (data) are recorded in plural on each header of the optical-magnetic disc.
FIG. 1 shows an example of recording the address information on two parts of each header.
In FIG. 1, a first sector 1 is constituted by the header HD (HD1) and data region DA (DA1). The header HD1 is constituted by a sector mark M, an address mark AM1, an address information data AD1 whose head position is indicated by the address mark AM1, an address mark AM2 which is separated by a predetermined number of bytes from an end position of the address information data AD1, and address information data AD2 whose head position is indicated by the address mark AM2. The address information data AD1 have the same content as that of the address information data AD2.
Then, a previously proposed header information detecting apparatus starts a data read operation upon detection of at least one of the address information data AD1 or AD2 at a time of, for example, a data reading.
On the other hand, when data is to be written, the header information detecting apparatus starts a data write operation only upon the detection of both address information data AD1 and AD2.
However, in the previously proposed header information detecting apparatus, it is not possible to detect a start position of the subsequent address information data AD2 since the start position cannot be identified in a case when, for example, a slight defect or irregularity occurs on the address mark AM2. Therefore, the previously proposed header information detecting apparatus cannot carry out the write operation even though no defect is present on the address mark AM1, address information data AD1 and AD2.
Referring to FIG. 2, in another, a previously proposed sector mark detector of the header information detecting apparatus detects the sector mark M and, thereafter, a count of a number of bytes is started upon detection of the sector mark M.
When the counted number of bytes reaches a number corresponding to a head position of the address mark AM1, a previously proposed window signal generator thereof generates a window signal which provides an open state (usually at an "H (high)" level) for a predetermined interval of time (corresponding to a fixed window time width). The window signal which is in the open state is supplied to a previously proposed address mark detector to initiate the detection of the address mark AM1. When the address mark AM1 is detected, the address information data AD1 is read on the basis of the detected address mark AM1. Next, when the counted number of bytes reaches the number corresponding to the head position of the address mark AM2, the detection of the address mark AM2 is initiated in accordance with the window signal derived from the window signal generator. When the address mark AM2 is detected, the address information data AD2 is read on the basis of the detected address mark AM2.
In this way, in the previously proposed address mark detector, the detection operation of the address mark is controlled in accordance with the window signal.
The previously proposed address mark detector fixes the time width of the window signal which is in the open state to a marginal width wider than a width of data in which the address mark can be present, with a time axis deviation between the window signal and address mark when the sector mark cannot be detected taken into consideration.
However, if the time width during which the window signal is in the open state is wider than the width of data in which the address mark can be present, a possibility of erroneously recognizing the address mark becomes increased. If the address mark is erroneously recognized, a correct recognition of the address information cannot be made in the previously proposed header information detecting apparatus and the accuracy of reading the data on the magnetic recording medium can be reduced.